Debilitating chronic pain afflicts between 70-90 million Americans with headache (24 million), backache (23 million), arthritis (40 million), and millions more with other pathologies, diseases, and injuries. Debilitating chronic pain costs Americans over $50 billion annually, including $900 million on over the counter analgesics, and billions more in lost production.
Chronic pain consists of multiple causes and manifestations that are treated by many different healthcare practitioners. Pain can usually be linked to causes such as injury or disease but can also be idiopathic with no diagnosable origin. Pain may be characterized simplistically by the following types:
1. Nociceptive Pain
This is the common pain that signals tissue irritation, impending injury, or actual injury. Nociceptors (i.e, pain receptors) in the affected area are activated, which then transmit signals via the peripheral nerves and the spinal cord to the brain. Complex spinal reflexes (withdrawal) may be activated, followed by perception, cognitive and affective responses, and possibly voluntary action. The pain is typically perceived as related to the specific stimulus (hot, sharp, etc.) or with an aching or throbbing quality. Visceral pain is a subtype of nociceptive pain. It tends to be paroxysmal and poorly localized, as opposed to somatic pain that is more constant and well localized. Nociceptive pain is usually time limited—arthritis is a notable exception—and tends to respond well to treatment with opioids (e.g., morphine).
2. Neuropathic Pain
Neuropathic (neural injury) pain is the result of a malfunction somewhere in the nervous system. The site of the nervous system injury or malfunction can be either in the peripheral or central nervous system. The pain is often triggered by an injury, but this injury may not clearly involve the nervous system, and the pain may persist for months or years beyond the apparent healing of any damaged tissues. In this setting, pain signals no longer represent ongoing or impending injury. The pain frequently has burning, lancinating, or electric shock qualities. Persistent allodynia—pain resulting from a nonpainful stimulus, such as light touch—is also a common characteristic of neuropathic pain. Neuropathic pain is frequently chronic, and tends to have a less robust response to treatment with opioids.
3. Psychogenic Pain
Psychogenic pain describes those rare situations where it is clear that no somatic disorder is present. It is universal that psychological factors play a role in the perception and complaint of pain. These psychological factors may lead to an exaggerated or histrionic presentation of the pain problem, but even in these circumstances, it is rare that the psychological factors represent the exclusive etiology of the patient's pain.
4. Mixed Category Pain
In some conditions, the pain appears to be caused by a complex mixture of nociceptive, neuropathic and/or psychogenic factors. An initial nervous system dysfunction or injury may trigger the neural release of inflammatory mediators and subsequent neurogenic inflammation. For example, migraine headaches probably represent a mixture of neuropathic and nociceptive pain. As another example, myofascial pain is probably secondary to nociceptive input from the muscles, but the abnormal muscle activity may be the result of neuropathic conditions. Chronic pain, including chronic myofascial pain, may cause the development of ongoing representations of pain within the central nervous system that are independent of signals from the periphery. This is called the centralization or encephalization of pain.
A large variety of treatments exist for pain. However, for various reasons, many patients continue to suffer with their painful conditions. In addition, many treatments suffer from a variety of disadvantages. For instance, existing surgical procedures are highly invasive. What is needed are more and better treatment options, including systems and methods to effectively and efficiently deliver drugs directly to specific areas of the brain affecting pain, possibly in combination with electrical stimulation, to relieve pain. In combination with drug and/or electrical stimulation of specific areas of the brain affecting pain, means of sensing a patient's condition and responding with the appropriate stimulation is also needed.